How to display F-stop limits in viewport?
Depth of Field Utilities Addon

This Add-on allows to visualize the Depth of Field limits (near limit &amp; far limit) in the viewport:



Once the Add-on is enabled, select the camera, open up the Properties region of any 3d View (N), scroll all the way down to the bottom and enable the visualization. From there you can fine tune all relevant properties of the selected camera:



Notice that you can disable the instructions to hit Esc in the Add-on Preferences if needed.

In addition to that, you can also set the focus distance by using the 3d Cursor: 



Further reading: How do I set focus range?